'I am reaching the end of my ordeal," says Umberto Eco when we meet. Happily, I don't take this personally. Eco – philosopher, semiotician, novelist, bibliophile and all-round brainbox – has been on a 20-day global tour to promote his new novel, The Prague Cemetery, and says at times he has barely known what city he was in.

Eco, who will be 80 in January, doesn't look too bad for his ordeal. His rotundity means he sits a little awkwardly in his chair, but he is a lively, playful interviewee, chewing on a small cigar throughout. He gave up smoking them eight years ago, but still likes to have one in his mouth and hopes some of the nicotine gets through. He has a rasping voice and an idiosyncratic take on English. The conversation occasionally breaks down when I use expressions he doesn't quite grasp. He misunderstands when I ask him whether The Prague Cemetery is, as some critics have suggested, a "return to form": for him, form is a sporting rather than a literary term.

Anyway, we battle on. The elephant in our cramped little room is that the new book is not a return to form, whether literary or sporting. Set in the second half of the 19th century and following the fortunes of master forger, murderer and general bad egg Simone Simonini, who manages to have a hand in most of the great events of that period (Italian unification, Franco-Prussian war, Paris Commune, Dreyfus affair), it is a wearying read. In English at least. Perhaps it sparkles in Italian.

Whether or not it is a return to form, it is certainly a return to Eco's favourite subject – conspiracies. Simonini is presented as the originator of the Protocols of the Elders of Zion, the early 20th-century fake text that purported to detail a Jewish conspiracy aimed at world domination. Following its publication in Russia in 1903, it was widely read and believed, despite being shown to have been plagiarised from fictional sources. Hitler quoted it extensively, and even now its poison circulates. Eco pieces together what little is known of the origins of the text, and offers Simonini, an amoral Italian living in Paris, as the originator of the most toxic of all forgeries.

Conspiracies in general, and the Protocols in particular, have been recurrent themes in Eco's work, notably in his second novel, Foucault's Pendulum, where as a joke three nondescript book editors concoct a grand conspiracy that comes to take over their lives. Why do the Protocols preoccupy him? "As a scholar I am interested in the philosophy of language, semiotics, call it what you want, and one of the main features of the human language is the possibility of lying. A dog doesn't lie. When it barks, it means there is somebody outside." Animals do not lie; human beings do. "From lies to forgeries the step is not so long, and I have written technical essays on the logic of forgeries and on the influence of forgeries on history. The most famous and terrible of those forgeries is the Protocols."

Eco says it is not conspiracies that attract him, but the paranoia that allows them to flourish. "There are many small conspiracies, and most of them are exposed," he says. "But the paranoia of the universal conspiracy is more powerful because it is everlasting. You can never discover it because you don't know who is there. It is a psychological temptation of our species. Karl Popper wrote a beautiful essay on that, in which he said it started with Homer. Everything that happens in Troy was plotted the day before on the top of Olympus by the gods. It's a way not to feel responsible for something. That's why dictatorships use the notion of universal conspiracy as a weapon. For the first 10 years of my life I was educated by fascists at school, and they used a universal conspiracy – that you, the Englishman, the Jews and the capitalists were plotting against the poor Italian people. For Hitler it was the same. And Berlusconi has spent all his electoral campaigns speaking of the double conspiracy of the judges and the communists. There are no more communists around, even if you look for them with a lamp, but for Berlusconi they were there trying to take over."

Sean Connery and Christian Slater in The Name of the Rose. Photograph: Allstar/Cinetext Collection/Sportsphoto/Allstar/Cinetext Collection

He probably does not intend to elide Hitler and Berlusconi, but nor is he a fan of Italy's recently departed prime minister. Eco has always been a prominent figure on the political left, and has opposed Berlusconi since his first stint as PM in the mid-1990s. He is pleased that the great partygoer has fallen, but warns against writing him off, suggesting he may try to return after the elections due in 2013.

"Berlusconi is a genius in communication," says Eco. "Otherwise he would never have become so rich. From the beginning he identified his target – middle-aged people who watch television. Young people do not watch television; they are on the internet. The people who support Berlusconi are 50- and 60-year-old ladies and retired people, who, in a country with an ageing population, make a powerful electoral force. So even some of his famous blunders may be blunders for me and you, but probably for the provincial 60-year-old lady or gentlemen they are not. His appeal was 'pay less taxes'. When the premier says you are right not to pay taxes, you are pleased."

How could a culture as intellectual and artistic as Italy's have elected such a buffoon? "Berlusconi was strongly anti-intellectual," he says, "and boasted that he hadn't read a novel for 20 years. There was a fear of the intellectual as a critical power, and in this sense there was a clash between Berlusconi and the intellectual world. But Italy is not an intellectual country. On the subway in Tokyo everybody reads. In Italy, they don't. Don't evaluate Italy from the fact that it produced Raphael and Michelangelo."

Eco's new book has been attacked by some for regurgitating an antisemitic text, but he argues that the Protocols can easily be found on the internet and that "weak readers" who misunderstand his purpose will be misled elsewhere. "You are not responsible for perverse readings of your book," he says. "Catholic priests said don't give Madame Bovary to a young girl to read because she might be seduced by adultery."

Does it bother him that the half-dozen novels he has produced since The Name of the Rose propelled him to fictional fame in the early 1980s have had a mixed reception? "You are always shocked by how different critics' opinions are," he says. "I think a book should be judged 10 years later, after reading and re-reading it. I was always defined as too erudite and philosophical, too difficult. Then I wrote a novel that is not erudite at all, that is written in plain language, The Mysterious Flame of Queen Loana, and among my novels it is the one that has sold the least. So probably I am writing for masochists. It's only publishers and some journalists who believe that people want simple things. People are tired of simple things. They want to be challenged."

Eco had a distinguished 30-year career in the academic world, with sidelines making cultural TV programmes and working as an editor in Milan, before The Name of the Rose. Why did he feel the need to add fiction to an already overloaded CV? In part, he says, it was accident. A friend asked him to write a short detective novel for a new series she was preparing. He told her that if he did, it would be set in the middle ages and would have to be 500 pages. That was too big for the proposed series, but the idea had been planted in his mind (or, as he prefers, his belly), and a publishing phenomenon was born. Even without her intervention, however, he implies that he would eventually have written novels. The notion of poisoning a monk appealed to him, and he already had a list of monkish names filed away in his drawer for possible use.

"I have always had a narrative impulse," he says. "I wrote stories and beginnings of novels at the age of 10 or 12. I then satisfied my taste for narrative by writing essays. All my researches have the structure of a whodunit." One of his professors pointed out that even his doctoral thesis on Thomas Aquinas had that structure, with the conclusion teasingly arrived at after a long process of divination. "I recognised he was right, and that I was right, and that research must be done this way. I satisfied my narrative impulse when my kids were small by telling them stories, and then when they were grown up I felt the need to write fiction. It happened to me as it happens to people when they fall in love. 'Why did you fall in love that day, that month, with that person? Are you crazy? Why?' You don't know. It happens."

The Name of the Rose made Eco's reputation as a novelist, but it has also proved difficult to match. "Sometimes I say I hate The Name of the Rose," he admits, "because the following books maybe were better. But it happens to many writers. Gabriel García Márquez can write 50 books, but he will be remembered always for Cien Años de Soledad [One Hundred Years of Solitude]. Every time I publish a new novel, sales of The Name of the Rose go up. What is the reaction? 'Ah, a new book of Eco. But I have never read The Name of the Rose.' Which, by the way, costs less because it is in paperback." He laughs, as he does frequently. Eco's great virtue is that he is an intellectual who doesn't take himself too seriously. Life, like fiction, is a wonderful game.

It is claimed that he called the film of The Name of the Rose a travesty, but that seems unlikely. He says only that a film cannot do everything a book can. "A book like this is a club sandwich, with turkey, salami, tomato, cheese, lettuce. And the movie is obliged to choose only the lettuce or the cheese, eliminating everything else – the theological side, the political side. It's a nice movie. I was told that a girl entered a bookstore and seeing the books said: 'Oh, they have already made a book out of it.'" More laughter.

The Name of the Rose sold – and continues to sell – by the bucketload. It made him rich, famous, sought after. But he chose to carry on teaching at Bologna university, and to keep up his academic work. His bibliography of non-fiction works on language, culture and belief is vast and forbidding. Hidden behind Eco the novelist and Eco the performer is a serious philosopher and literary critic.

It is often said that he constructs his novels out of other books. The Prague Cemetery both explores the 19th-century novels that were plagiarised in the Protocols, and is structured like one. Alexandre Dumas is the presiding spirit, in particular his novel Joseph Balsamo, and intertextuality the name of Eco's fictional game. He has adored books since he was a child, growing up in the town of Alessandria in northern Italy with not very bookish "petit bourgeois" parents but a grandmother who loved reading. He read voraciously and still does. His two libraries, at the homes he shares with his German-born wife Renate Ramge in Milan and Rimini, contain 50,000 books, including 1,200 rare titles.

He has called books "the corridors of the mind" and recently co-wrote an extended love letter to the printed text called This is Not the End of the Book. But that does not make him a digital counter-revolutionary. Indeed, to save having to carry a bag full of books, on this trip he has instead brought along an iPad with 30 titles downloaded. He nevertheless stands by his contention that this is not the end of the book. Reading devices are fine for long journeys and have advantages for reference books, but committed readers will always crave physicality – "not just Peter Pan but my Peter Pan", as he puts it.

The fact that he can accommodate everything from illuminated manuscripts to iPads is typical. He is optimistic, eclectic, eternally young, interested in everything, as at home discoursing on Peanuts as he is on Proust. I ask him how he will be remembered – as novelist, critic or polymath? "I leave it up to you," he says. "Usually a novelist has a longer-lasting life than an academic, unless you are Immanuel Kant or John Locke. Illustrious thinkers of 50 years ago have already been forgotten."

So is he resigned to being remembered for The Name of the Rose rather than his contribution to semiotics? "At the beginning," he says, "I had the impression that my novels had nothing to do with my academic interests. Then I discovered that critics found many connections, and the editors of the Library of Living Philosophers decided that my novels had to be taken into account as a philosophical contribution. So I surrender. I accept the idea that they match. Evidently I am not a schizophrenic."